The intercommunication between devices (hereinafter, terminals) with communication facility, such as personal digital assistants (PDAs), notebook PCs, and mobile phones, wire communications through the intermediary of cables, has conventionally been carried out through the intermediary of cables. Examples of such wire communications include RS-232C and USB (Universal Serial Bus).
Meanwhile, a near field wireless communication technology for wireless communication between terminals has been proposed. Typical examples of the near field wireless communication technology include IEEE 802.11, IEEE 802.15, and Bluetooth®.
When a communications network (hereinafter, network) is constructed using the near field wireless communication technology, a terminal for maintaining synchronization on the network is required. Such a terminal is generally termed master or coordinator, but hereinafter the terminal is referred to as master terminal. The master terminal communicate with other terminals so that a one-to-many network is constructed. This type of network is disclosed by, for instance, Japanese Laid-Open Patent Application No. 2002-232433 (Tokukai2002-232433, published on Aug. 16, 2002).
The master terminal has to be chosen in consideration of the reach of radio waves. FIGS. 18(a) and 18(b) show a case that the limited reach of radio waves hinders the construction of a wireless network. Provided that terminals A-D are configured as in FIG. 18(b) and the terminal A is the master terminal, radio waves from the terminal A do not reach the terminal D so that the terminal D cannot participate in the network.
On the other hand, when the terminal B is set as the master terminal as in FIG. 18(b), radio waves from the terminal B reach all of the remaining terminals A, C, and D, so that all of the terminals A-D can participate in the network. In this manner, the user has to grasp the physical relationships between all terminals, in order to select an appropriate terminal as the master terminal.
Furthermore, the master terminal also has to be chosen in consideration of obstacles for wireless communications. FIGS. 19(a) and 19(b) shows a case that an obstacle hinders the construction of a wireless network. Provided that terminals A-D are configured as in FIG. 19(a) and an obstacle exists between the terminals B and D, when the terminal B is the master terminal, radio waves from the terminal B cannot reach the terminal D because of the obstacle, so that the terminal D cannot participate in the network.
On the other hand, when the terminal C is the master terminal as in FIG. 19(b), radio waves from the terminal C reach all of the remaining terminals A, B, and D, so that all of the terminals A-D can participate in the network. In this manner, the user has to grasp the physical relationships between all terminals, in order to select an appropriate terminal as the master terminal.
Furthermore, when the terminals are mobile devices such as mobile phones and PDAs, it is assumed that the terminals are on the move and hence the user cannot properly select the master terminal.
To solve this problem, “Method of Operating Network at Breakaway from Master” disclosed by Japanese Laid-Open Patent Application No. 2002-111689 (Tokukai 2002-111689; published on Apr. 12, 2002) teaches that, when constructing a network of a plurality of communication devices by Bluetooth®, a backup master manages the network if a communication device functioning as a master of the network is not available.
This method includes a stage (a) where coupled information is transmitted from at least one slave out of many slaves existing within a network, a stage (b) of deciding the priority of at least one slave out of the many slaves, used as backup masters based on the above coupled information transmitted in case that it breaks away from a network master, and a stage (c) of broadcasting the priority information of the above decided backup masters to at least one other slave out of many slaves stated above.
According to this method, when the network master is broken away, an appropriate backup master is chosen in accordance with the coupled information having been transmitted, so that the network is managed by this chosen backup master.
However, the method of Japanese Laid-Open Patent Application No. 2002-111689 is effective only when the network master is broken away. Furthermore, when a plurality of one-to-many networks are connected with each other, it is impossible to chose an appropriate backup master so that the network cannot properly be constructed. This complicates the network structure and causes problems such as the increase of overall power consumption of the network due to the existence of many unnecessary master terminals and the reduction of communication speed owing to the increase of retransmission caused by frequent collision of packets.